In his closing submission to the royal commission, counsel assisting Richard Beasley SC said that by taking social and economic factors into consideration when setting environmental flows, the MDBA had erred.

"The Murray-Darling Basin Authority has misinterpreted the Water Act, not in a minor way, not in an unimportant way, in a crucial way," he said.

"That's not only error, or worse than error, it's a massive one with regrettable consequences for the lawfulness of that part of the Basin Plan."